In-situ fiber characterization using nonlinear skirt measurement

ABSTRACT

A system includes a processor communicatively coupled to an Amplifier Stimulated Emission (ASE) source and an optical receiver, wherein the processor is configured to cause transmission of one or more shaped ASE signals, from the ASE source, on an optical fiber, obtain received spectrum of the one or more shaped ASE signals from the optical receiver connected to the optical fiber, and characterize the optical fiber based in part on a nonlinear skirt and/or center dip depth in the received spectrum of the one or more shaped ASE signals. The one or more shaped ASE signals can be formed by the ASE source communicatively coupled to a Wavelength Selective Switch (WSS) that is configured to shape ASE from the ASE source to form the one or more shaped ASE signals with one or two or multiple peaks and with associated frequency.

FIELD OF THE DISCLOSURE

The present disclosure generally relates to fiber optic systems. Moreparticularly, the present disclosure relates to systems and methods forin-situ fiber characterization using nonlinear skirt measurement.

BACKGROUND OF THE DISCLOSURE

In optical systems, signal transmission performance is significantlyimpacted by fiber dispersion and nonlinearity. Chromatic dispersion,i.e., β₂, and fiber nonlinear coefficient, i.e., γ, are among the mostcritical fiber characteristics for photonic line system link budget,performance modeling, and optimization. The current fiber nonlinearitymeasurement can be with a high-power signal having a narrow linewidth orby backing out from a Stimulated Raman Scattering (SRS) measurement. Thedispersion measurement, currently, is commonly implemented by measuringthe phase delay difference of modulated signals on differentwavelengths. For example, dispersion can also be measured by thedifferential time delay between two wavelengths such as an OpticalService Channel (OSC) wavelength at 1510 nm and an Optical Time DomainReflectometer (OTDR) wavelength at 1625 nm.

Disadvantageously, Continuous Wave (CW) signals are single polarizationsources by definition. This causes measurement issues in the presence ofPolarization Dependent Gain (PDG), Differential Group Delay (DGD), andPolarization Mode Dispersion (PMD) which is significant in real systems.In addition, the practical implementation using CW source(s) fordispersion/nonlinear measurement requires a coherent transponder,transceiver, modem, etc. This requires equipped modules as well asend-to-end communication. This may not be available at or beforeturn-up. Also, the dispersion measurement using two wavelengths such as1510 nm and 1625 nm only measures average dispersion at 1568 nm. Asoptical systems continue to push the limit of bandwidth over fiber, itis critical to get accurate measurements over the entire signal band.

BRIEF SUMMARY OF THE DISCLOSURE

In an embodiment, a system includes a processor communicatively coupledto an Amplifier Stimulated Emission (ASE) source and an opticalreceiver, wherein the processor is configured to cause transmission ofone or more shaped ASE signals, from the ASE source, on an opticalfiber, obtain received spectrum of the one or more shaped ASE signalsfrom the optical receiver connected to the optical fiber, andcharacterize the optical fiber based in part on one or more of anonlinear skirt and a center dip depth in the received spectrum of theone or more shaped ASE signals. The one or more shaped ASE signals canbe formed by the ASE source communicatively coupled to a WavelengthSelective Switch (WSS) that is configured to shape ASE from the ASEsource to form the one or more shaped ASE signals with one or more peaksand with associated frequency. The one or more shaped ASE signals canhave two distinct peaks at the transmission with a significant dip at acenter frequency, and the received spectrum of the one or more two-peakASE signals has much less of a dip that the center dip depth. Theprocessor can be further configured to determine a fiber type based on asignature of the one or more of the nonlinear skirt and the center dipdepth in the received spectrum of the one or more shaped ASE signals.The optical fiber can be characterized to determine chromaticdispersion, β₂, and fiber nonlinear coefficient, γ.

The chromatic dispersion, β₂, and the fiber nonlinear coefficient, γ,can be characterized by a measurement of a shape of the one or more ofthe nonlinear skirt and the center dip depth as a function of signalwavelength. The one or more shaped ASE signals can include a pluralityof shaped ASE signals with a first set of shaped ASE signals utilized todetermine launch power for every span in a section to yield an optimumcenter dip depth, and a second set of shaped ASE signals that sweep atdifferent frequencies across a signal band to determine a correspondingcenter dip depth at the different frequencies. The optical fiber can becharacterized based in part on a shape of the one or more of thenonlinear skirt and the center dip depth and a separate differentialdelay measurement, to determine chromatic dispersion, β₂, and fibernonlinear coefficient, γ. The optical fiber can be characterized basedin part on a shape of the one or more of the nonlinear skirt and thecenter dip depth and a separate Stimulated Raman Scattering (SRS)measurement, to determine chromatic dispersion, β₂, and fiber nonlinearcoefficient, γ. The optical fiber can be characterized based in part ona shape of the one or more of the nonlinear skirt and the center dipdepth and a Least Mean Square (LMS) fit, to determine chromaticdispersion, β₂, and fiber nonlinear coefficient, γ.

In another embodiment, a method includes causing transmission of one ormore shaped Amplifier Stimulated Emission (ASE) signals, from an ASEsource, on an optical fiber; obtaining received spectrum of the one ormore shaped ASE signals from the optical receiver connected to theoptical fiber; and characterizing the optical fiber based in part on oneor more of a nonlinear skirt and a center dip depth in the receivedspectrum of the one or more two-peak ASE signals. The one or more shapedASE signals can be formed by the ASE source communicatively coupled to aWavelength Selective Switch (WSS) that is configured to shape ASE fromthe ASE source to form the one or more shaped ASE signals with one ormore peaks and with associated frequency. The one or more shaped ASEsignals can have two distinct peaks at the transmission with asignificant dip at a center frequency, and the received spectrum of theone or more two-peak ASE signals has much less of a dip that the centerdip depth. The method can further include determining a fiber type basedon a signature of the one or more of the nonlinear skirt and the centerdip depth in the received spectrum of the one or more shaped ASEsignals.

The optical fiber can be characterized to determine chromaticdispersion, β₂, and fiber nonlinear coefficient, γ. The one or moreshaped ASE signals can include a plurality of shaped ASE signals with afirst set of shaped ASE signals utilized to determine launch power forevery span in a section to yield an optimum center dip depth, and asecond set of shaped ASE signals that sweep at different frequenciesacross a signal band to determine a corresponding center dip depth atthe different frequencies. The optical fiber can be characterized basedin part on a shape of the one or more of the nonlinear skirt and thecenter dip depth and a separate differential delay measurement, todetermine chromatic dispersion, β₂, and fiber nonlinear coefficient, γ.The optical fiber can be characterized based in part on a shape of theone or more of the nonlinear skirt and the center dip depth and aseparate Stimulated Raman Scattering (SRS) measurement, to determinechromatic dispersion, β₂, and fiber nonlinear coefficient, γ. Theoptical fiber can be characterized based in part on a shape of the oneor more of the nonlinear skirt and the center dip depth and a Least MeanSquare (LMS) fit, to determine chromatic dispersion, β₂, and fibernonlinear coefficient, γ.

In a further embodiment, an Optical Add/Drop Multiplexing (OADM) nodeincludes a Wavelength Selective Switch (WSS) system communicativelycoupled to at least a first optical fiber and a second optical fiber; anAmplifier Stimulated Emission (ASE) source connected to the WSS system;a pre-amplifier connected to the WSS system and the first optical fiber;an Optical Channel Monitor (OCM) connected at least to an output of thepre-amplifier; and a processor configured to obtain received spectrum ofone or more shaped ASE signals from the OCM, wherein the one or moreshaped ASE signals are transmitted over the first optical fiber, andcharacterize the first optical fiber based in part on a nonlinear skirtshape and a center dip depth in the received spectrum of the one or moreshaped ASE signals.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure is illustrated and described herein withreference to the various drawings, in which like reference numbers areused to denote like system components/method steps, as appropriate, andin which:

FIG. 1A is a graph of an optical signal and associated signal broadeningover 100 km in Lambda Shifted (LS) fiber utilizing a one-peak signal,and FIG. 1B is a graph of another optical signal utilizing a two-peaksignal;

FIG. 2A is a graph of measured results for received spectrum in LS fiberover 100 km, and FIG. 2B is a graph of measured results for receivedspectrum in Non-Dispersion Shifted Fiber (NDSF) over 40 km;

FIG. 3A is a graph of simulated results for received spectrum inTruewave (TW) Classic fiber, FIG. 3B is a graph of simulated results forreceived spectrum in LS fiber, FIG. 3C is a graph of simulated resultsfor received spectrum in NDSF fiber, FIG. 3D is a graph of simulatedresults for received spectrum in Large Effective Area Fiber (LEAF), FIG.3E is a graph of simulated results for received spectrum in enhancedLEAF (eLEAF), and FIG. 3F is a graph of simulated results for DispersionShifted Fiber (DSF);

FIG. 4A is a graph of measurement of center gap depth versus frequencyversus β₂ for LS fiber, and FIG. 4B is a graph of a measurement ofcenter gap depth versus frequency versus β₂ for NDSF fiber;

FIG. 5 is a graph of a simulation of center dip depth versus β₂ withdifferent launching power;

FIG. 6 is a graph of a simulation of center dip depth versus β₂ withdifferent gamma, γ;

FIG. 7 is a network diagram of an optical section with associatedequipment for providing an in-situ nonlinear skirt measurement;

FIG. 8 is a flowchart of a measurement process which can be implementedin the optical section of FIG. 7;

FIG. 9A is a flowchart of a data analysis process which also uses adifferential delay measurement, FIG. 9B is a flowchart of a dataanalysis process which uses an SRS measurement, and FIG. 9C is aflowchart of a data analysis process which sweeps dispersion, dispersionslope and γ;

FIG. 10 is a block diagram of a system for characterizing opticalamplifiers utilizing the two-peak ASE signal;

FIG. 11A is a graph of optical spectrum in the system before subtractingASE, FIG. 11B is a graph of optical spectrum in the system aftersubtracting ASE, and FIG. 11C is a graph of FWM level versus signalcenter frequency; and

FIG. 12 is a flowchart of a process for in-situ fiber characterization.

DETAILED DESCRIPTION OF THE DISCLOSURE

The present disclosure relates to systems and methods for in-situ fibercharacterization using nonlinear skirt measurement. The presentdisclosure characterizes fiber dispersion and nonlinearity based on thespectral shape of an Amplified Stimulated Emission (ASE) signal. Ofnote, the ASE signal is more like a coherent, dual-polarizationmodulated signal than a single polarization CW source. The ASE signalcan explore all states of polarization which averages these effectsresulting in a more stable and accurate measurement. Also, the ASEsignal is available from common infrastructure in a photonic line systemwithout requiring additional equipment, and this can be utilized on aper section basis prior to system turn-up. For example, next-generationphotonic line systems utilize ASE for channel holders and/or cangenerate ASE via amplifiers. An ASE source can be shaped by a WavelengthSelective Switch (WSS) at Tx. At a corresponding Rx, the nonlinearproduct can be characterized by the nonlinear skirt, i.e., spectralshape of the ASE signal. In an example, a two-peak Tx signal spectralshape is designed using an ASE source and transmitted over the fiber. Ofnote, other shapes are also contemplated such as a one-peak Tx signal.At a corresponding Rx, the nonlinear product can be characterized bycenter dip depth, i.e., the relative power difference between the signalpeak and the valley of the overlap area of the nonlinear skirt betweenthe two peaks. As a result, measurement depends on relative powerbetween the peak and valley of the two-peak signal. Absolute poweraccuracy and power monitor Wavelength Dependent Loss (WDL) are notcritical requirements for the measurement.

The present disclosure is characterized as in-situ due to utilizingexisting common infrastructure in an optical networking system; nospecialized hardware is required. The present disclosure measuresdispersion and nonlinearity within the signal band (e.g., the C-bandsuch as between about 1528 nm and 1565 nm, and/or the L-band such asbetween about 1565 nm and 1625 nm) rather than relying on out-of-bandmeasurements and extrapolation. The present disclosure utilizes arelative power measurement, such that measurement can be easily carriedout and post-processing is not complicated.

The accurate characterization of fiber dispersion and nonlinearity willhelp model the exact link performance, which is crucial for link budgetand performance optimization. This is a key part of a “plug and play”approach for optical control. A fiber-type determination is currently amanual procedure and has been shown to be a source of system issues inmany networks where there is often mis-provisioned fiber type. Thisdisclosure removes the manual effort of fiber type identification andprovisioning and the associated potential manual error. Furthermore,fiber-type does not accurately characterize the fiber. Even within eachfiber type, there is a range of dispersion and nonlinear coefficient. Itis also common in real systems to have mixed fiber types within a singlespan. This disclosure gives an appropriate value, averaged over thenonlinear-length, for the key performance parameters which can be usedin optical control an optimization resulting in better performance andhigher capacity.

When an optical signal propagates in a nonlinear medium such as opticaltransmission fiber, its spectrum will be broadened due to thecombination of fiber nonlinearity and dispersion. The broadened spectralshape shows a distinct signature for fiber with differentcharacteristics. Consequently, β₂ and γ can be backed out throughmeasurement and then calculated. This disclosure proposes a process thatcharacterizes β₂ and γ by measuring the broadened spectrum of a shapedASE signal.

FIG. 1A is a graph 10A of an optical signal and associated signalbroadening over 100 km in Lambda Shifted (LS) fiber illustrating aone-peak signal. FIG. 1B is a graph 10B similar to FIG. 1A illustratinga two-peak signal. A spectral shape of the Tx signal is illustrated byline 12 in the graph 10A, 10B, and the Rx signal after a 100 km LS spanis illustrated by a line 14 in the graph 10A, 10B. Of note, the graphs10A, 10B illustrate the Tx signal and the Rx signal on the same graphfor illustration purposes to highlight the difference in the spectralshapes based on transmission over an optical fiber. The nonlinearbroadening effect due to the transmission will result in a skirt shapeon the Rx signal spectrum, a.k.a. nonlinear skirt 18. That is, when anoptical signal is transmitted over fiber, the signal spectrum will bebroaden due to the combination of fiber nonlinearity and dispersion, andthe received signal shape is different after different type of fiberswith different characteristics. Therefore, an LMS fit and the like canbe performed to determine the various fiber parameters.

In FIG. 1B, it is observed that the gap between the two peaks, marked asa center dip depth 16 in the graph 10B, becomes shallower aftertransmission due to the broadening effect. It is discovered as outlinedin this disclosure that β₂ and γ can be characterized by measuringcenter dip depth as a function of signal wavelength.

Of note, the present disclosure performs fiber characterization (e.g.,fiber type determination, β₂ and γ measurements, etc. based on sendingan ASE signal that is spectrally shaped and measuring the receivedspectral shape, in particular, the nonlinear skirt. In an example, atwo-peak signal as shown in FIG. 1B is utilized where an ASE signal froman ASE source is shaped to form the two peaks. The two-peak signalsimplifies measurement and post-processing by providing a relativemeasurement between the peak and valley of the two peaks. Those ofordinary skill in the art will appreciate it is possible to characterizefiber based on the nonlinear skirt shape of a one-peak signal as shownin FIG. 1A based on LMS fit of the shape of the nonlinear skirt.Further, the present disclosure contemplates other shapes includingmultiple peaks.

FIG. 2A is a graph of measured results for received spectrum in LS fiberover 100 km, and FIG. 2B is a graph of measured results for receivedspectrum in Non-Dispersion Shifted Fiber (NDSF) over 40 km. FIG. 3A is agraph of simulated results for received spectrum in Truewave (TW)Classic fiber, FIG. 3B is a graph of simulated results for receivedspectrum in LS fiber, FIG. 3C is a graph of simulated results forreceived spectrum in NDSF fiber; FIG. 3D is a graph of simulated resultsfor received spectrum in Large Effective Area Fiber (LEAF), FIG. 3E is agraph of simulated results for received spectrum in enhanced LEAF(eLEAF), and FIG. 3F is a graph of simulated results for DispersionShifted Fiber (DSF).

Specifically, the measured/experimental results in FIG. 2 and thesimulated results in FIG. 3 illustrate the change of the center dipdepth 16 sweeping the signal across the C-band. Distinct signatures areobserved for different fiber type with different characteristics. Ofnote, fiber type can be identified from the change of the center dipdepth 16 over wavelength.

Furthermore, an important phenomenon is discovered that given the fiberto be transmitted, the normalized center dip depth over wavelength doesnot change with different signal launching power. This can be explainedby a Gaussian Noise (GN) model, where the nonlinear product of fibertransmission is expressed as ^([1])

$\begin{matrix}{{G_{NLI}\left( {f,\beta_{2},\alpha,L_{S},\gamma,P_{0}} \right)} = {\frac{16}{27}\gamma^{2}P_{0}^{3}{\int_{- \infty}^{\infty}{\int_{- \infty}^{\infty}{{{\frac{1 - {e^{{- 2}\alpha L_{s}}e^{j4\pi^{2}{\beta_{2}{({f_{1} - f})}}{({f_{2} - f})}}}}{{2\alpha} - {j\; 4\pi^{2}{\beta_{2}}\left( {f_{1} - f} \right)\left( {f_{2} - f} \right)}}}^{2} \cdot {g_{Tx}\left( f_{1} \right)}}{g_{Tx}\left( f_{2} \right)}{g_{Tx}\left( {f_{1} + f_{2} - f} \right)}df_{2}df_{1}}}}}} & (1)\end{matrix}$

where β₂ is the fiber dispersion, γ is the nonlinear coefficient, α isthe fiber loss parameter, L_(s) is the span length, g_(Tx)(⋅) is thenormalized power spectral density of the transmitted signal, P₀ is itslaunching power. This is taken from [1] A. Carena, V. Curri, G. Bosco,P. Poggiolini and F. Forghieri, “Modeling of the Impact of NonlinearPropagation Effects in uncompensated Optical Coherent TransmissionLinks,” in Journal of Lightwave Technology, vol. 30, no. 10, pp.1524-1539, May 15, 2012.

The normalized shape of G_(NLI)(f) is

$\begin{matrix}{{g_{NLI}\left( {f,\beta_{2},\alpha,L_{S}} \right)} = {\int_{- \infty}^{\infty}{\int_{- \infty}^{\infty}{{{\frac{1 - {e^{{- 2}\alpha L_{s}}e^{j4\pi^{2}{\beta_{2}{({f_{1} - f})}}{({f_{2} - f})}}}}{{2\alpha} - {j\; 4\pi^{2}{\beta_{2}}\left( {f_{1} - f} \right)\left( {f_{2} - f} \right)}}}^{2} \cdot {g_{Tx}\left( f_{1} \right)}}{g_{Tx}\left( f_{2} \right)}{g_{Tx}\left( {f_{1} + f_{2} - f} \right)}df_{2}df_{1}}}}} & (2)\end{matrix}$which only depends on the term within the absolute operator wheng_(Tx)(⋅) is fixed. The Tx signal spectrum with the two-peak spectralshape designed by this invention has no signal power between the twopeaks. Therefore, the Power Spectral Density (PSD) in the center gap atthe Rx side contains only nonlinear products. When PSD of the in bandnonlinear product is negligible compare with signal PSD, the center dipdepth, i.e., the delta of PSD between the peak and valley of thetwo-peak signal, can be written as

$\begin{matrix}{\left( {\beta_{2},\ P_{0},\ \gamma,\ L_{S},\ \alpha} \right) = {{{10\mspace{14mu}\log\mspace{14mu} 10\left( \frac{g_{RX}\left( f_{c} \right)}{g_{NLI}\left( {{f = 0},\beta_{2},L_{s},\alpha} \right)} \right)} + {10\mspace{11mu}\log\mspace{14mu} 10\left( {\frac{16}{27}\gamma^{2}P_{0}^{2}} \right)}} = {{\overset{\sim}{D}{{epth}\left( {\beta_{2},L_{S},\alpha} \right)}} + \ {A\left( {\gamma,P_{0}} \right)} + C}}} & (3)\end{matrix}$where f_(c) is the relative center frequency of the each of the twopeaks, as marked in FIG. 1B. The first term in Eq (3) is the normalizedcenter dip depth, denoted as {tilde over (D)}epth(β₂,L_(s),α); thesecond term is the offset, denoted as A(γ,P₀), and C=10log10(16/27).

FIG. 4A is a graph of measurement of center gap depth versus frequencyversus β₂ for LS fiber, and FIG. 4B is a graph of a measurement ofcenter gap depth versus frequency versus β₂ for NDSF fiber. Given thefiber span under test, i.e., L_(eff) and γ are approximately constantover wavelength, the normalized center dip depth as a function of β₂,{tilde over (D)}epth|_(L) _(eff) (β_(s)), remains unchanged withdifferent P₀, while the offset, A(P₀), changes twice as fast as P₀.Note, the corresponding β₂ at each frequency on the top X-axis of FIGS.4A and 4B is computed from measured dispersion and dispersion slope at1550 nm.

FIG. 5 is a graph of a simulation of center dip depth versus β₂ withdifferent launching power. FIG. 6 is a graph of a simulation of centerdip depth versus β₂ with different gamma, γ. Note, the shape of the β₂versus the center dip depth trace remain approximately unchanged whencenter dip depth is greater than about 15 dB. The center dip depth traceversus β₂ versus measure can provide the fiber type and dispersionslope. The shape of the center dip depth versus β₂ changes at differentfiber lengths, but is unchanged with different gamma, γ at the samefiber length. For detection, the shape of the center dip depth versus β₂can be stored at various different fiber lengths for simple comparisons.The simulation results in FIG. 5 demonstrate that, with fixed γ andL_(eff), {tilde over (D)}epth|_(L) _(eff) (β₂) remains approximatelyunchanged with different P₀, when the absolute value of Depth is largerthan ˜15 dB. Depth gets distorted with high P₀, when the in bandnonlinear product is no longer negligible. FIG. 6 demonstrates that withfixed P₀ and L_(eff), {tilde over (D)}epth|_(L) _(eff) (β₂) remainsapproximately unchanged, while absolute value of Depth changes twice asfast as the change of γ.

Accordingly, the relative center dip depth change over wavelength can bea distinct indicator of the type of fiber (e.g., NSDF, LS, TWc, LEAF,eLEAF, DSF, etc.). No absolute power information is required;

FIG. 7 is a network diagram of an optical section 50 with associatedequipment for providing an in-situ nonlinear skirt measurement. Theoptical section 50 is a segment in an optical network between OpticalAdd/Drop Multiplexer (OADM) nodes 52, 54. The optical section 50 can bereferred to as an Optical Multiplex Section (OMS), and one aspect ofeach optical section 50 is the spectral load is identical over theentire section. A real implementation of an optical network can includemultiple optical sections 50 in a mesh, ring, linear, hub and spoke,etc. architecture. The in-situ nonlinear skirt measurement can beperformed on a per optical section basis. The optical section 50 canalso include intermediate optical line amplifier nodes 56. Further, apractical implementation of the optical section 50 includes two opticalfibers 58, 60 for bidirectional communication. The in-situ nonlinearskirt measurement is performed on each optical fiber 58, 60 separately.

The OADM nodes 52, 54 include a Wavelength Selective Switch (WSS) 62that faces the optical fibers 58, 60. The WSS 62 forms an optical degreethat faces the optical fibers 58, 60. In this example, a single degreeis illustrated at each of the OADM nodes 52, 54. Of course, practicalimplementations may include multiple degrees, each facing a differentoptical section 50. The WSS 62 is configured to add/drop spectrumto/from the degrees and/or locally. Each OADM node 52, 54 includes apost-amplifier 64 on the transmit side and a pre-amplifier 66 on thereceive side. The amplifiers 64, 66 can be Erbium-Doped Fiber Amplifiers(EDFAs). Also, Raman amplifiers may be used as well in addition toEDFAs. The OADM nodes 52, 54 also include an Optical Channel Monitor(OCM) 68 (a.k.a. Optical Power Monitor (OPM), etc.) which is an opticalreceiver connected (e.g., by a tap) to an output of each of theamplifiers 64, 66. The OCM 68 can have two receivers to simultaneouslymonitor each of the optical fibers 58, 60 or a switch to allow a singlereceiver to monitor one of the optical fibers 58, 60 at a time.

The OADM nodes 52, 54 also can include an ASE source 70 coupled to theWSS 62. In newer optical line systems, the ASE source 70 can be used tofill unused spectrum to reduce power optimization time. Here, the ASEsource 70 provides so-called channel holders used to fill the opticalspectrum on the optical section 50 so that it always appears to have afull-fill configuration. Such an approach significantly reduces capacitychange time.

Also, the OADM nodes 52, 54 can include other components 72 such as anOTDR, an OSC, a polarimeter, and a processor. Again, the components 72are in-situ, i.e., part of the OADM nodes 52, 54. In an embodiment, thecomponents 72 can provide a differential delay measurement, an SRSmeasurement, a fiber 58, 60 effective length (L_(eff)) measurement, etc.For example, these measurements are described in commonly-assigned U.S.patent application Ser. No. 15/986,396, filed May 22, 2018, and entitled“Optical fiber characterization measurement systems and methods,” thecontents of which are incorporated by reference herein. Further, theprocessor can be used to obtain the measurement data and perform variousdata analyses described herein. Also, the intermediate optical lineamplifier nodes 56 include in-line optical amplifiers 72.

As described herein, the in-situ fiber nonlinear skirt measurement canbe performed with the ASE source 70 and the WSS 62 causing a two-peaksignal to be transmitted on the optical fibers 58, 60 and received bythe OCM 68. Referring back to FIGS. 1A and 1B, the graphs 10A, 10Billustrate the transmitted spectral shape in line 12. This spectralshape can be achieved through configuration of the ASE source 70 and theWSS 62. The received spectral shape in line 14 is received by the OCM 68with the corresponding center dip depth 16 a function of the fiber 58,60.

FIG. 8 is a flowchart of a measurement process 100. The measurementprocess 100 can be implemented in the optical section 50. Themeasurement process 100 starts with an initial scan at 1550 nm. At thebeginning of a network section, e.g., at the upstream OADM, themeasurement process 100 includes setting up the WSS 62 to shape the ASEsource 70 to create the two-peak signal at 1550 nm (step 102).

For every span i=1˜N in the optical section, starting with i=1 (step104), the measurement process 100 includes setting the opticalamplifiers in a power mode for all spans (this power mode setting onlyneeds to be done once, not necessarily for each iteration) and settingsignal launching power at a reference power level, P0 _(ref) for span i,for example P0 _(ref)=15 dBm, and setting the rest of the spans at amuch lower launching power, for example (P0 _(ref)−15) (step 106). Themeasurement process 100 includes reading the OPM 68 at the downstreamOADM, and recording the center dip depth of the received signal asDepth_(ref)|_(span=i) (step 108). The span count is incremented andsteps 104-110 are repeated until the end of the section (step 110).

The signal broadening effect characterized by Depth_(ref)|_(span=i) ismainly generated by span i with high signal launching power. The purposeof steps 104-110 is to find the launching power for every span to yieldthe center dip depth around an optimum center dip depth, Depth_(opt).Depth_(opt) is found when the PSD of the nonlinear product at the centergap is much higher than line EDFA ASE noise, while the correspondingin-band nonlinear product is still negligible compared to the signal.Depth_(opt) depends on the width of the signal and the gap of the twopeaks. For example, when both the signal and gap width is 50 GHz,Depth_(opt)=15 dB. Since the absolute level of Depth changes twice asfast as P0, the launching power for span i=1˜N is computed byP0|_(span=i) =P0_(ref)+(Depth_(ref)|_(span=i)−Depth_(opt))/2(dBm)  (4)

The next step is to sweep the signal through the signal band at X pointswith the optimum signal launching power P0|_(span=i) from Eq.(4). In theupstream OADM, for every wavelength to be tested (step 112), themeasurement process 100 includes setting up the WSS/ASE source to createthe two-peak signal at wavelength x, x=1˜X(step 114).

For every span i=1˜N in section (step 116), the measurement process 100includes setting the optical amplifiers in power mode for all spans(this power mode setting only needs to be done once, not necessarily foreach iteration), and setting launch power of span i to P0|_(span=i),such that the center dip depth due to the signal broadening effect ofspan i is around the pre-defined Depth_(opt). For the rest of the spansj=1˜N, j≠i, set launch power toP0|_(span=j)=P0_(ref)+(Depth_(ref)|_(span=j)=(Depth_(opt)+20))/2(dBm)  (5)such that the center dip depth due to the signal broadening effect ofspan=j is about 20 dB less than Depth_(opt), and is deemed as negligiblein the signal spectrum at the end of section (step 118).

For every span i and wavelength x, the measurement process 100 includesmeasuring the center dip depth of the received signal with the OCM atthe downstream OADM and recorded as Depth_(meas)|_(span=i,wvl=x) (step120). Steps 112-120 are repeated for all spans and wavelengths (steps122, 124). Finally, the measurement process 100 has all required datameasurements and can proceed to data analysis which is described inprocesses 200A, 200B, 200C.

FIG. 9A is a flowchart of a data analysis process 200A which also uses adifferential delay measurement, FIG. 9B is a flowchart of a dataanalysis process 200B which uses an SRS measurement, and FIG. 9C is aflowchart of a data analysis process 200C which sweeps dispersion,dispersion slope and γ. Each of the processes 200A, 200B, 200C can beperformed in a processor at the OADM nodes 52, 54, in a managementsystem, or the like.

In FIG. 9A, the data analysis process 200A utilizes dispersion data at1568 nm from a differential delay measurement such as from an OSC at1510 nm and an OTDR at 1625 nm. This dispersion data is utilized inaddition to data from the measurement process 100.

The data analysis process 200A, for each span i=1˜N at each measuredwavelength, x=1˜X (step 202), includes loading measurement results fromcenter dip depth measurement Depth_(meas)|_(wvl=x,span=i), for each spani=1˜N at each measured wavelength, x=1˜X (step 204). The data analysisprocess 200A includes getting the measured dispersion at 1568 nm fromthe differential delay measurement, denoted as β₂|_(wvl=1568,span=i)(step 208); as well as the effective length, L_(eff), from an OTDRmeasurement (step 206). Then, the data analysis process 200A includesloading the pre-computed normalized center dip depth as a function β₂ atthe measured L_(eff)|_(span=i), denoted as {tilde over (D)}epth|_(L)_(eff) (β₂).

Next, the data analysis process 200A includes scaling {tilde over(D)}epth|_(L) _(eff) (β₂) to Depth_(meas)|_(wvl=x,span=i), byextrapolating Depth_(meas)|_(wvl=x,span=i) to Depth|_(wvl=1568,span=i)(step 210) and finding A to satisfy Eq. (6) (step 212):Depth_(meas)|_(wvi=1568,span=1)={tilde over (D)}epth|_(L) _(eff)(β₂|_(wvi=1568,span=i))+A+c  (6)

Next, the data analysis process 200A, from Eq.3, includes determining γis byγ=10^(0.1*(A/2″P0|) ^(span=i) ⁾  (7)and β₂ of the fiber at each measured wavelength, β₂|_(wvl=x,span=i), byβ₂|_(wvi=x,span=i)={tilde over (D)}_(epth)|_(L) _(eff)⁻¹(Depth_(meas)|_(wvl=x,span=i) −A−c)  (8)Where {tilde over (D)}epth|_(L) _(eff) ⁻¹( ) is the inverse function of{tilde over (D)}epth|_(L) _(eff) ( ) (step 214). The data analysisprocess 200A loops through every span in the section (step 216).

In FIG. 9B, the data analysis process 200B incorporates an SRSmeasurement. The data analysis process 200B, for each span i=1˜N at eachmeasured wavelength x=1˜X (step 220); starts with loading results fromcenter dip depth measurement, Depth_(meas)|_(wvl=x,span=i) (step 222),loading γ|_(span=i) from SRS measurement (step 226), and pre-computed{tilde over (D)}epth|_(L) _(eff) (β₂) at OTDR measured L_(eff)|_(span=i)(step 224). The scaling factor A can be found by (step 228)a=2P0|_(span=i)+20log10(γ|_(span=i))  (9)

Then, β₂ of the fiber at each measured wavelength, β₂|_(wvi=x,span=i),is found by Eq (8) (step 230). The data analysis process 200B loopsthrough every span in the section (step 232).

In FIG. 9C, the data analysis process 200C sweeps dispersion, dispersionslope and γ to scale {tilde over (D)}epth|_(L) _(eff) (β₂) to a guessedDepth_(guess)(γ,β₂(λ)) based on Eq.(3). The data analysis process 200C,for each span i=1˜N at each measured wavelength x=1˜X (step 240), startswith loading results from center dip depth measurement,Depth_(meas)|_(wvl=x,span=i) (step 242), and pre-computed {tilde over(D)}epth|_(L) _(eff) (β₂) at OTDR measured L_(eff)|_(span=i) (step 244).The data analysis 200C sweeps dispersion, dispersion slope and γ toscale {tilde over (D)}epth|_(L) _(eff) (β₂) to a guessedDepth_(guess)(γ,β₂(λ)) (step 246). By comparing Depth_(guess)(γ,β₂(λ))with the measured Depth_(meas)|_(wvl=x), γ|_(span=i) andβ₂|_(wvl=x,span=i) are found at Least Mean Square (LMS) fit (steps 248,250). The data analysis process 200C loops through every span in thesection (step 252).

In an embodiment, the measurement technique described herein with atwo-peak ASE signal can be used to characterize optical amplifiers. Forexample, next-generation optical amplifiers with higher output power canhave tighter Four-Wave Mixing (FWM) specifications. The variousmeasurement techniques described herein work well to characterize thiseffect. For example, conventionally, the FWM specification of amplifierswas verified in the factory and lab using a specified measurementtechnique with two CW lasers (which also means single polarization) thathad to be polarization controlled (scanned) to search for a peakinteraction, with fixed spacing between the CW lasers which could not bewidened because of polarization evolution, such that the walk-off couldnot be measured. This approach was only valid for FWM tones of 50 GHznarrow single polarization sources. This is not representative of theNonlinear (NL) impact on wider, dual-polarization, coherent transceiversor modems.

The measurement technique employed here uses two-peaks of ASE and hasthe following benefits 1) ASE more accurately represents spectrallyshaped dual-polarization coherent signals, 2) this eliminates the strongpolarization hole burning that contaminates the CW approach, 3) the peakseparation can be varied to get a direct measurement of walk-off, and 4)it is easy to create multiple peaks to get a full-fill representativeG(length) FWM measurement.

FIG. 10 is a block diagram of a system 300 for characterizing opticalamplifiers 302, 304 utilizing the two-peak ASE signal. The system 300includes the ASE source 70 which is connected to the optical amplifiers302, 304, such as via WSSs 306, 308 (or a single WSS). An OpticalSpectrum Analyzer (OSA) 310 can be selectively connected to an output ofthe amplifier 302 and the amplifier 304.

A measurement process utilizing the system 300 can include creating atwo-peak ASE signal with the ASE source 70 and the WSSs 306, 308,sending the two-peak ASE signal to the amplifiers 302, 304, andmeasuring the outputs of the amplifiers 302, 304 with the OSA 310. Here,and in the section 50, the WSSs can be configured to set attenuation toeven the power of the two-peaks in the two-peak ASE signal.

First, the optical amplifier 302 can have its Total Output Power (TOP)adjusted to normalize signal power to a first frequency tested (e.g.,187000 GHz) and an OSA trace can be recorded. Next, the opticalamplifier 304 can have its Total Output Power (TOP) adjusted tonormalize signal power to a first frequency tested (e.g., 187000 GHz)and an OSA trace can be recorded after calibration. The OSA trace can berecorded at the output of the optical amplifier 304 with stepping signalfrequent and TOP of either amplifier 302, 304.

FIG. 11A is a graph of optical spectrum in the system 300 beforesubtracting ASE, FIG. 11B is a graph of optical spectrum in the system300 after subtracting ASE, and FIG. 11C is a graph of FWM level versussignal center frequency.

FIG. 12 is flowchart of a process 400 for in-situ fibercharacterization. The process 400 includes causing transmission of oneor more shaped Amplifier Stimulated Emission (ASE) signals, from an ASEsource, on an optical fiber (step 402); obtaining received spectrum ofthe one or more shaped ASE signals from the optical receiver connectedto the optical fiber (step 404); and characterizing the optical fiberbased in part on one or more of a nonlinear skirt and a center dip depthin the received spectrum of the one or more two-peak ASE signals (step406). The process 400 can also include determining a fiber type based ona signature of the one or more of the nonlinear skirt and the center dipdepth in the received spectrum of the one or more shaped ASE signals(step 408). In another embodiment, the process 400 can have the step 402be completed separately and the obtaining step 404 can be performedresponsive to another device causing the transmission. For example, theprocess 400 here could be performed by a management system or the likethat receives the spectrum of the one or more shaped ASE signals. Thetransmission or the causing of such transmission can be performed by anoptical network element or node. The received spectrum by the opticalnetwork element or node can be provided to the processor or otherapparatus implementing the process 400.

The one or more shaped ASE signals can be formed by the ASE sourcecommunicatively coupled to a Wavelength Selective Switch (WSS) that isconfigured to shape ASE from the ASE source to form the one or moreshaped ASE signals with one or more peaks and with associated frequency.The one or more shaped ASE signals can have two distinct peaks at thetransmission with a significant dip at a center frequency, and thereceived spectrum of the one or more two-peak ASE signals has much lessof a dip that the center dip depth. The optical fiber can becharacterized to determine chromatic dispersion, β₂, and fiber nonlinearcoefficient, γ. The chromatic dispersion, γ₂, and the fiber nonlinearcoefficient, γ can be characterized by a measurement of a shape of theone or more of the nonlinear skirt and the center dip depth as afunction of signal wavelength.

The one or more shaped ASE signals can include a plurality of shaped ASEsignals with a first set of shaped ASE signals utilized to determinelaunch power for every span in a section to yield an optimum center dipdepth, and a second set of shaped ASE signals that sweep at differentfrequencies across a signal band to determine a corresponding center dipdepth at the different frequencies. The optical fiber can becharacterized based in part on a shape of the one or more of thenonlinear skirt and the center dip depth and a separate differentialdelay measurement, to determine chromatic dispersion, β₂, and fibernonlinear coefficient, γ. The optical fiber can be characterized basedin part on a shape of the one or more of the nonlinear skirt and thecenter dip depth and a separate Stimulated Raman Scattering (SRS)measurement, to determine chromatic dispersion, β₂, and fiber nonlinearcoefficient, γ. The optical fiber can be characterized based in part ona shape of the one or more of the nonlinear skirt and the center dipdepth and a Least Mean Square (LMS) fit, to determine chromaticdispersion, β₂, and fiber nonlinear coefficient, γ.

It will be appreciated that some embodiments described herein mayinclude one or more generic or specialized processors (“one or moreprocessors”) such as microprocessors; Central Processing Units (CPUs);Digital Signal Processors (DSPs): customized processors such as NetworkProcessors (NPs) or Network Processing Units (NPUs), Graphics ProcessingUnits (GPUs), or the like; Field Programmable Gate Arrays (FPGAs); andthe like along with unique stored program instructions (including bothsoftware and firmware) for control thereof to implement, in conjunctionwith certain non-processor circuits, some, most, or all of the functionsof the methods and/or systems described herein. Alternatively, some orall functions may be implemented by a state machine that has no storedprogram instructions, or in one or more Application Specific IntegratedCircuits (ASICs), in which each function or some combinations of certainof the functions are implemented as custom logic or circuitry. Ofcourse, a combination of the aforementioned approaches may be used. Forsome of the embodiments described herein, a corresponding device inhardware and optionally with software, firmware, and a combinationthereof can be referred to as “circuitry configured or adapted to,”“logic configured or adapted to,” etc. perform a set of operations,steps, methods, processes, algorithms, functions, techniques, etc. ondigital and/or analog signals as described herein for the variousembodiments.

Moreover, some embodiments may include a non-transitorycomputer-readable storage medium having computer readable code storedthereon for programming a computer, server, appliance, device,processor, circuit, etc. each of which may include a processor toperform functions as described and claimed herein. Examples of suchcomputer-readable storage mediums include, but are not limited to, ahard disk, an optical storage device, a magnetic storage device, a ROM(Read Only Memory), a PROM (Programmable Read Only Memory), an EPROM(Erasable Programmable Read Only Memory), an EEPROM (ElectricallyErasable Programmable Read Only Memory), Flash memory, and the like.When stored in the non-transitory computer-readable medium, software caninclude instructions executable by a processor or device (e.g., any typeof programmable circuitry or logic) that, in response to such execution,cause a processor or the device to perform a set of operations, steps,methods, processes, algorithms, functions, techniques, etc. as describedherein for the various embodiments.

Although the present disclosure has been illustrated and describedherein with reference to preferred embodiments and specific examplesthereof, it will be readily apparent to those of ordinary skill in theart that other embodiments and examples may perform similar functionsand/or achieve like results. All such equivalent embodiments andexamples are within the spirit and scope of the present disclosure, arecontemplated thereby, and are intended to be covered by the followingclaims.

What is claimed is:
 1. A system comprising: a processor communicativelycoupled to an Amplifier Stimulated Emission (ASE) source and an opticalreceiver, wherein the processor is configured to cause transmission ofone or more shaped ASE signals, from the ASE source, into an opticalfiber of an optical section between Optical Add/Drop Multiplexer nodes,obtain a spectrum of the one or more shaped ASE signals from the opticalreceiver connected to the optical fiber, and characterize the opticalfiber based in part on one or more of a nonlinear skirt and a center dipdepth in the received spectrum of the one or more shaped ASE signals. 2.The system of claim 1, wherein the one or more shaped ASE signals areformed by the ASE source communicatively coupled to a WavelengthSelective Switch (WSS) that is configured to shape ASE from the ASEsource to form the one or more shaped ASE signals with one or more peaksand with associated frequency.
 3. The system of claim 1, wherein the oneor more shaped ASE signals have two distinct peaks at the transmissionwith a significant dip at a center frequency, and the received spectrumof the one or more two-peak ASE signals has much less of a dip that thecenter dip depth.
 4. The system of claim 1, wherein the processor isfurther configured to determine a fiber type based on a signature of theone or more of the nonlinear skirt and the center dip depth in thereceived spectrum of the one or more shaped ASE signals.
 5. The systemof claim 1, wherein the optical fiber is characterized to determinechromatic dispersion, β₂, and fiber nonlinear coefficient, γ.
 6. Thesystem of claim 5, wherein the chromatic dispersion, β₂, and the fibernonlinear coefficient, γ, are characterized by a measurement of a shapeof the one or more of the nonlinear skirt and the center dip depth as afunction of signal wavelength.
 7. The system of claim 1, wherein the oneor more shaped ASE signals includes a plurality of shaped ASE signalswith a first set of shaped ASE signals utilized to determine launchpower for every span in a section to yield an optimum center dip depth,and a second set of shaped ASE signals that sweep at differentfrequencies across a signal band to determine a corresponding center dipdepth at the different frequencies.
 8. The system of claim 1, whereinthe optical fiber is characterized based in part on a shape of the oneor more of the nonlinear skirt and the center dip depth and a separatedifferential delay measurement, to determine chromatic dispersion, β₂,and fiber nonlinear coefficient, γ.
 9. The system of claim 1, whereinthe optical fiber is characterized based in part on a shape of the oneor more of the nonlinear skirt and the center dip depth and a separateStimulated Raman Scattering (SRS) measurement, to determine chromaticdispersion, β₂, and fiber nonlinear coefficient, γ.
 10. The system ofclaim 1, wherein the optical fiber is characterized based in part on ashape of the one or more of the nonlinear skirt and the center dip depthand a Least Mean Square (LMS) fit, to determine chromatic dispersion,β₂, and fiber nonlinear coefficient, γ.
 11. A method comprising: causingtransmission of one or more shaped Amplifier Stimulated Emission (ASE)signals, from an ASE source, into an optical fiber of an optical sectionbetween Optical Add/Drop Multiplexer nodes; obtaining a spectrum of theone or more shaped ASE signals from the optical receiver connected tothe optical fiber; and characterizing the optical fiber based in part onone or more of a nonlinear skirt and a center dip depth in the receivedspectrum of the one or more two-peak ASE signals.
 12. The method ofclaim 11, wherein the one or more shaped ASE signals are formed by theASE source communicatively coupled to a Wavelength Selective Switch(WSS) that is configured to shape ASE from the ASE source to form theone or more shaped ASE signals with one or more peaks and withassociated frequency.
 13. The method of claim 11, wherein the one ormore shaped ASE signals have two distinct peaks at the transmission witha significant dip at a center frequency, and the received spectrum ofthe one or more two-peak ASE signals has much less of a dip that thecenter dip depth.
 14. The method of claim 11, further comprisingdetermining a fiber type based on a signature of the one or more of thenonlinear skirt and the center dip depth in the received spectrum of theone or more shaped ASE signals.
 15. The method of claim 11, wherein theoptical fiber is characterized to determine chromatic dispersion, β₂,and fiber nonlinear coefficient, γ.
 16. The method of claim 11, whereinthe one or more shaped ASE signals includes a plurality of shaped ASEsignals with a first set of shaped ASE signals utilized to determinelaunch power for every span in a section to yield an optimum center dipdepth, and a second set of shaped ASE signals that sweep at differentfrequencies across a signal band to determine a corresponding center dipdepth at the different frequencies.
 17. The method of claim 11, whereinthe optical fiber is characterized based in part on a shape of the oneor more of the nonlinear skirt and the center dip depth and a separatedifferential delay measurement, to determine chromatic dispersion, β₂,and fiber nonlinear coefficient, γ.
 18. The method of claim 11, whereinthe optical fiber is characterized based in part on a shape of the oneor more of the nonlinear skirt and the center dip depth and a separateStimulated Raman Scattering (SRS) measurement, to determine chromaticdispersion, β₂, and fiber nonlinear coefficient, γ.
 19. The method ofclaim 11, wherein the optical fiber is characterized based in part on ashape of the one or more of the nonlinear skirt and the center dip depthand a Least Mean Square (LMS) fit, to determine chromatic dispersion,β₂, and fiber nonlinear coefficient, γ.
 20. An Optical Add/DropMultiplexing (OADM) node comprising: a Wavelength Selective Switch (WSS)system communicatively coupled to at least a first optical fiber and asecond optical fiber; an Amplifier Stimulated Emission (ASE) sourceconnected to the WSS system; a pre-amplifier connected to the WSS systemand the first optical fiber; an Optical Channel Monitor (OCM) connectedat least to an output of the pre-amplifier; and a processor configuredto obtain a spectrum of one or more shaped ASE signals from the OCM,wherein the one or more shaped ASE signals are transmitted over thefirst optical fiber, and characterize the first optical fiber based inpart on a one of a nonlinear skirt and a center dip depth in thereceived spectrum of the one or more shaped ASE signals.